Seasons is a permanent feature that updates throughout the year. In Seasons you can purchase Costumes for your Birds using Gems, and earn rewards by playing levels with Style Score. Your Style Score progress can be accessed in the second tab of the Seasons hub.
There are two types of rewards available: free and premium. The free rewards are given on every second tier up until tier 30. With the paid Premium Pass you will get new rewards on every tier right up until tier 40! You will also get access to rare costumes for every bird. When you purchase the Premium Pass you will instantly receive all the premium rewards from the previously completed tiers.
Seasons will change through the year and you will notice that different Costumes have changing Multipliers, these Multipliers are applied to your Style Score at the end of levels to help you earn Seasons rewards faster. At the start of a new Season your Style Score rewards will reset.